The invention relates to a charge transfer device (CTD) eliminating the background level of a detected signal provided with an input circuit and a transfer circuit formed in a semiconductor substrate, the said signal being detected by means of the input circuit, which comprises:
a charge injection source connected to a charge emitter, which supplies a quantity of charge, PA1 a control electrode, which permits that the injection source injects this quanity of charge into a sorage potential well of the CTD, whose depth is controlled by a storage electrode, PA1 the storage well being subdivided into an evacuation well and an output well separated from each other by the separation potential produced by a separation electrode, which permits of retaining in the evacuation well a quantity of charge to be evacuated and of retaining in the output well a quantity representative of the useful signal, which is read by the transfer circuit by means of a transfer electrode and is then conducted to the output of the CTD connected to processing members, while the charge in the evacuation well can be evacuated by means of a zero reset transistor. PA1 a floating electrode connected to the separation electrode in order that the separation potential controls the background level, this potential being determined during a calibration operation by means of a reference charge emitter, which charges from the storage well the reference well situated under the floating electrode, the dopings under the floating electrode and under the separation electrode, respectively, being obtained in order that the potential wells situated under each of these respective electrodes have different depths, PA1 and an insulation electrode, which insulates during the measuring operations the reference well from the output well, the latter being discharged beforehand, the useful signal being read by means of the reference well, the floating electrode being charged by means of a transistor connected to a reference potential. PA1 firstly a calibration of the evacuation well is effected from a reference radiation, PA1 by charging during a given period from the storage well a reference well situated under a floating electrode in order to modify its electric potential and to control the potential of the separation electrode to which it is connected so that the evacuation well is insulated from the output well by a calibrated separation potential, the doping under the floating electrode and under the separation electrode, respectively, being obtained in order that the potential wells situated under each of these respective electrodes have different depths, PA1 by means of an insulation electrode, the output well is insulated from the reference well, which is held in the charged state, PA1 the evacuation well and the output well are emptied with respect to their charge, PA1 secondly, the measurement on the scene to be analysed is carried out by introducing the charge produced by the charge emitter into the injection source, which first fills the evacuation well and then the output well, the quantity of charge of the output well being transferred to the transfer circuit of the CTD so as to be conducted to processing elements.
It further relates to a detection apparatus or a detection structure comprising several charge transfer devices connected individually to a charge emitter. These devices may be diodes made of Cd.sub.x Hg.sub.(1-x) Te in order to obtain an infrared detection.
An invention of this kind is known from the publication "Hybrid Infrared Focal Plane Arrays" by Kuen Chow, J. P. Rode, D. H. Seib, J. D. Blackwell, I.E.E.E. Transactions on Electron Devices, Vol. ED-29, No. 1, p. 3, January 1982. This publication describes a focal plane hybrid network for the detection of radiation by means of photovoltaic detectors and bars of CTD's. This network operates at low temperatures to detect infrared radiation. Nevertheless the dynamic range defined as the signal-to-background level ratio S/B is comparatively small at the level of the detector and it has proved to be indispensable to increase this S/B ratio by means of a CTD, whose input circuit(s) is (are) specially designed to attenuate the component relative to the background level of the signal of the detector. Therefore, the signal originating from the detector acts upon the injection source of the CTD to supply electric charge introduced into a storage potential well, which may be subdivided into two wells, i.e. an evacuation well and an output well, in order to isolate in the output well a quantity of charge compatible with the transfer possibilities of the CTD. This is effected by means of a separation electrode, to which a determined and fixed electric voltage is externally applied. This voltage is the same for all the CTD's constituting a detection apparatus comprising several CTD's. The charge contained in the output well is then poured into the transfer circuit of the CTD by means of a transfer electrode, whose potential can be determined to carry out either an operation of separating the excess charge or a simple transfer.
This device permits of increasing the dynamic range, but nevertheless remains insufficient in practice, for the same potential is applied to all the CTD's constituting a detection apparatus with regard to the separation electrodes on the one hand and to the transfer electrodes on the other hand.
Now, each injection source is connected individually to a photovoltaic detector. It is clear that the background level, viewed by each CTD, will be greatly different because of the addition of all the dispersions connected with the chain formed by the photovoltaic detector, the injection source of the CTD, the dimensions of the electrodes, the doping of the semiconductor in the potential well and the threshold voltages.
These dispersions can reach a value of 30% to 50% of the maximum storage capacity of the transfer zone of the CTD.
Each radiation detector analyses a point of a scene and the injected signal then processed by the CTD is subsequently used to recover a picture. These erratic dispersions associated with each path produce a strongly degraded final picture.
Likewise, according to this prior art, in which the said division of the charge and/or the charge is separated, it is not really possible to suppress the influence of the background level.
Therefore, the invention has for its object to eliminate the unfavourable effect of this cumulation of the dispersions associated with each detector, to increase the picture quality and to suppress the background level.